JPS63228431A - Optical information pick-up device - Google Patents

Abstract

PURPOSE:To execute the miniaturization, low price, mass production and high reliability by using a lens Fourier transformation type hologram element by combining it with other optical element for an optical system. CONSTITUTION:An element 121 is a lens Fourier transformation type hologram to record a knife edge optical system, primary diffracted light is stopped by a focusing lens 23, namely, one side of the diffracted reproducing wave surfaces is Fourier-transformed to the close axis light of a lens 9 and the reproducing wave surface of the knife edge optical system occurs at the photoelectric converting surface of a two-divided photodetector 31. An axis epsilon of a hologram surface is coincident with a dividing boundary line direction X3 of the photodetector 31 (first photoelectric converting means), and even when the wavelength change of a light source 1 occurs, a focus control can be operated without trouble. A 2-divided boundary line Y3 direction of a photodetector 8 (second photoelectric converting means) is arranged in parallel to the track direction (tangent direction at reading point) of an optical disk 6, the far field of the pit is detected and a tracking control is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a pickup device for storing and reproducing optical information stored on an optical or magneto-optical medium such as an optical disk or an optical card.

Conventional technology Optical memory technology that uses bit-like patterns as a high-density, large-capacity storage medium is being put into practical use with its applications expanding to include digital audio discs, video discs, document file discs, and even data files. . The mechanism by which information is recorded and reproduced successfully with high reliability through a light beam focused on the micron order depends solely on the configuration for picking up the optical information, especially the optical system. The basic functions of the optical information pickup device (hereinafter abbreviated as OPU) are (1)
(11) focus control and pit signal detection of the optical system; and (11D)
The tracking control is roughly divided into three types.

These are realized by combining various optical systems and photoelectric conversion detection methods depending on the purpose and use. 6th
The figure is a schematic diagram showing an example of a conventional OPU. Normal T
A diverging wavefront (electric field: horizontally polarized wave) from a semiconductor laser light source 1 with a wavelength of about 800 nm that oscillates in E0° mode is converted into a parallel beam by a collimator lens 2, and a quarter-wave plate (% (lambda plate) 4 is selectively reflected.

The circularly polarized light wavefront that has passed through the Kλ plate is narrowed down to a spot of about 1 μm by the condenser lens system 6, reaches the optical disk medium surface θ, and irradiates a bit-like pattern. The light beam reflected and diffracted by the medium surface 6 travels in the opposite direction through the condensing lens system 5 again and passes through the quarter-wave plate 4, becoming a vertically polarized parallel beam, which passes through the polarizing beam splitter 3 and enters the prism. It is divided into two directions by a half mirror 7. One of the reflected lights passes through a condensing lens 9 and a cylindrical lens 1o that imparts astigmatism, enters a four-division seven-otodetector 11, and is converted into a focus control signal. The other transmitted light enters the two-split photodetector 8 for tracking control signal detection with the far field pattern unchanged.

Here, by combining the λλ plate 4 with the polarization beam splitter 3, the λλ plate 4 increases the efficiency of using the amount of light, and at the same time suppresses the return to the semiconductor laser, so that unnecessary noise does not increase in the signal light component. This is an ingenuity. but,
In the OPU for read-only discs, there is leeway in the light intensity design, and it is possible to omit the Kλ plate and polarizing beam splitter, and in particular, for miniaturization and cost reduction, parts can be omitted.

Compounding is being planned.

Problems to be Solved by the Invention However, even in a read-only OPU, it is necessary to construct a beam splitting means, a focus control means using astigmatism or the Knifezi method, and a tracking control means independently or in combination. The optical components conventionally used for this purpose, such as beam splitters, lenses, and prisms, are difficult to manufacture, assemble, and adjust in large quantities, and it is difficult to manufacture, assemble, and adjust them in large quantities. There was a problem.

The common reasons for these problems are: First, optical parts that require highly accurate flat or aspherical surfaces require many processes to achieve the desired processing, so production using pressing means is generally not possible. Second, it requires a lot of time and complicated inspection and measurement equipment to assemble and adjust a large number of parts to achieve a desired overall performance. Third, the parts are small. Since there are limits to the size of the optical system, there were also major constraints on the miniaturization of the entire optical system.

As a method for partially solving these problems, a technique has been developed in which, for example, the collimating lens 2 (or 9) shown in FIG. 6 is constructed of a Fresnel lens, and the lens is press-molded using a mold.

However, this does not reduce the number of parts, and components with a larger number of surfaces, such as the beam splitter, remain unreplaced. Furthermore, due to the limitations of processing accuracy, the lens 6, which requires the highest performance light focusing, cannot be replaced.

The above-mentioned reason can be solved by introducing an optical element having multiple functions, and an attempt has recently been reported to place a hologram element 21 close to the condenser lens 6 as shown in FIG.

((1) Mokuzai, Ono, Sugama, Ota; Autumn 1961 Proceedings of the Japan Society of Applied Physics, 30p-ZE-1, p. 227 (19
86). (2) Same; 22nd Micro-Optics Research Conference Lecture Paper;
vol. 4 (1986) p. 38) Conventionally, the wavelength range suitable for hologram recording (λ1: 4oO~500
(nm), and a near-infrared or red laser (λ2: ~800nm, 633nm) suitable as an OPU light source.
m ), significant aberrations occurred due to the lens action of the hologram, and it was difficult to correct them. Therefore, the hologram element is constructed at two points P1 and 2 using an optical system as shown in the same figure. This is a so-called lensless Fourier transform hologram in which interference fringes between P2 and reference light source R (actually, interference fringes between wavefronts 230 and 231 on one half of the hologram surface and interference fringes between wavefronts 230 and 232 on the remaining surface) are formed on the Mieg surface. The hologram element 21 is designed based on a system concept, and the hologram element 21 is divided into two parts 211 and 212 in order to have an effect equivalent to the "wedge prism method" or the "double knife method". realized by In this way, it is possible to create a hologram lens 21 that is free of aberration only for the design wavelength λ2 of the light source 1 used, and moreover, the aberration caused by slight variations in the spectral width of the light source is suppressed by the photodetector of the beam detector (photodetector) 22. Even if it appears on the conversion surface, the 4-divided photoelectric conversion surface 221°222.223.
The push-pull method using H.224 makes it possible to suppress fluctuations within a range that does not pose a practical problem. However, in FIG. 11, the pattern of the element 21 that can be written with an electron beam is
The technique is limited to simple patterns such as gratings and hyperbolic shapes, and no technique for forming more general hologram systems with high precision is disclosed at all. Also, a part of the coherent beam emitted from the light source 1 returns to the hologram element '-F-2.
1 and returns to the light emitting surface, "return light noise"
In addition, since the optical system is such that the coherent beam moves back and forth through the hologram element 21, there is a large amount of light loss (the amount of light taken out to the photodetector as a control beam is less than There were problems such as: That is, it has been difficult to simplify the optical system and reduce its cost without reducing the performance of the optical system.

The present invention provides a multifunctional hologram element that stably realizes focus and tracking control of an OPU. A simplified optical system can be constructed using a hologram element based on optical principles.

Differences between the conventionally disclosed hologram element for pickup and the hologram element according to the present invention will be explained in detail in the following explanation, but here, we will specifically explain the differences in terms of multiple functions. Let us compare and summarize the limitations of conventional elements seen from the above and the objectives of the present invention.

(1) Although both function as means for separating incident and reflected light, conventional hologram elements are used as light beams used to detect focus errors and tracking errors. ,
It is also limited to one specific optical system configuration. The present invention aims to realize a hologram element that basically replaces all the functions of conventionally developed optical systems such as the "astigmatism method" according to the needs of pickup design.

(2) Conventional hologram elements were configured as "lensless Fourier transform holograms" that minimized the lens function as focusing power, but there was a slight wavelength shift (Δλ = ±20 nm) from the design wavelength λ of the pickup light source.
A focus offset also occurs for the IIC, and it is necessary to provide a troublesome process of adjusting the position of the photodetector in the optical axis direction in order to adjust the wavelength shift due to the loft of the semiconductor laser. In the present invention, a hologram element can be designed and manufactured at any wavelength as a lens Fourier transform type that basically does not impart focusing power to the hologram element, and the above-mentioned position adjustment in the optical axis direction is not required.

(3) In conventional hologram elements, the beam reciprocates through the same element, so no matter how the diffraction efficiency is designed, there is a loss in the amount of light, and there are limitations in that optical noise returned to the light source cannot be suppressed. In the present invention, it is possible to design the hologram element so that the return beam is incident on the hologram element, thereby increasing the efficiency of using the amount of light.

Means for Solving the Problems The present invention is an optical information pickup device that solves the above-mentioned problems. a first optical system for converting the coherent beam into a beam; a transparent substrate having a thin film medium on a first surface that selectively reflects a predetermined polarized wavefront of the coherent beam; and a lens Fourier transform of the predetermined wavefront disposed on the back side of the substrate. hologram element, a quarter-wave plate, a second optical system that converges the coherent beam into a minute spot, and the coherent beam is reflected by a predetermined optical storage medium and returned through the second optical system. a third optical system that Fourier transforms a predetermined diffracted light from the hologram element in the optical path;
a first photoelectric conversion means disposed near the optical axis of the third optical system; and a second photoelectric conversion means for detecting zero-order diffracted light transmitted through the hologram element in the far field or air field of the minute spot. It is to be equipped.

Regarding the characteristics of the working lens Fourier transform hologram, see the literature ((3) Kanji memory using holography).

Kato, Fujito, Sato; Proceedings of the Institute of Image Electronics Engineers of Japan 79-0
4-1 (1979, 11) (4) 5 peaklere
duction in holography (speckle reduction in holography)...
...”.

M, Kato at al; Appl, Opt;
(Apply Obtex), 14 (1975) 109
As reported and analyzed in detail in 3), the track record of application to general image recording and reproducing optical systems ((at the time, ``Optical Kanji Editing System'' by Sato et al.; Institute of Electronics and Communication Engineers Study Group Materials, EC78- e3 (1978) 47), but in the present invention, as long as it does not pose a practical problem as a beam control means,
It is sufficient that Fourier transform is established for the wavefront near the optical axis of the reproduction optical system, and the lens used to reproduce the wavefront from the hologram element can be replaced with a collimating lens, or the hologram element can be simply irradiated with a convergent spherical wave. It is possible to obtain a desired reconstructed image on the optical surface.

The present invention solves the following problems by providing the above-described configuration.

(1) In the process of creating a hologram element, a hologram recording system that performs lens Fourier transformation of a predetermined wavefront such as an astigmatism wavefront or a Naifetsu optical system is used using a wavelength λ1 of a coherent light source suitable for hologram recording. 2) The optical system of the OPU uses a converging lens with almost no aberration for a given light source wavelength λ2 such as a semiconductor laser, or irradiates the hologram element with an equivalent convergent wavefront to achieve the desired focus and tracking control. can be formed on the photoelectric conversion surface (photodetector); It is possible to set the reference wave light source position of the hologram recording optical system so that the directions match, and to differentially detect a pair of conjugate images as necessary.
(→ One reconstructed image from the hologram element, for example, a focused beam corresponding to the diffracted wavefront from the knife beam, is used for focus control, and the zero-order transmitted light component from the hologram is
It can be used for direct tracking control in far-field or air-field positions, as well as
(5) Since the back surface of the hologram element substrate can be used as a reflective surface, it is also easy to form a multilayer dielectric thin film for polarization plane selection integrally.

Embodiment FIG. 1 shows a schematic configuration of an OPU device according to an embodiment of the present invention. 1 is a short wavelength semiconductor laser (wavelength λ2 = 800 nm) similar to that shown in Fig. 7, and 2 is a collimating lens (
Focal length f c = 201111), 120 is a multi-functional element equipped with a polarization selective reflection surface 122 and a hologram surface 121 on the front and back, and all incident light from the light source is reflected by the left C λ plate 4 and condensed. Lens 5 (numerical aperture NA=0.5
．． A minute spot of about 1 μmφ is formed on the bit surface of the optical disk e with a focal length fp=4×). The light beam reflected and diffracted by the bit surface passes through the lens 6 and the λλ plate 4 again, and then passes straight through the polarization selective reflection surface 122 with the polarization plane rotated by 900 degrees from the previous optical path, forming a hologram. The light is incident on the surface 121. The element 121 is a lens-Fourier transform hologram recording a Knifezi optical system as described below, and the first-order diffracted light is transmitted through the condensing lens 23 (
Focal length f2=2omi.

NA=o, 1), that is, one of the diffracted reproduced wavefronts is Fourier-transformed with respect to the paraxial ray of the lens 9, and the reproduced wavefront of the Naifetsu optical system described below is photoelectrically converted by the photodetector 31, which splits the reproduced wavefront into two. Occurs on the surface. On the other hand,
The 0th order diffracted light transmitted through the hologram surface 121 forms a far field pattern on the two-part photodetector 8. The knife optical system used for focus control here is as shown in FIG.
A coherent beam 13 with a wavelength λ1 that converges on a wavelength of The defocused beam images 181 and 183 appearing in the image and the minute spot image on the focal plane are converted. Furthermore, the axis ξ of the hologram surface (
The figure a) is aligned with the dividing boundary line direction x3 of the photodetector 31 (the tenth photoelectric conversion means) (the figure b), so that even if the wavelength of the light source 1 changes, the focus control can be operated without any problem. be. The two-division boundary line Y3 direction of the photodetector 8 (second photoelectric conversion means) is arranged parallel to the track direction (tangential direction at the reading point) of the optical disk 6, and tracking control is performed by detecting the far field of the bit. It will be done.

Fig. 2 is a conceptual diagram explaining the second embodiment of the present invention, and unlike Fig. 1, an astigmatic wavefront is recorded on the hologram surface, and is reproduced by reflected light from the optical disk. Quadrant photodetector 1 with a pair of conjugate wavefronts
1.17 (first photoelectric conversion means) as an astigmatism image. Similarly to the first embodiment, the 0th order diffracted light from the hologram surface enters a tracking control photodetector (second photoelectric conversion means) installed in the far field or in the air field. The photodetector 11.17 is integrated, with a small aperture 88 in the center.
Although the zero-order diffracted light is taken out to the detector 8 through the detector 11, the detector 8 may be arranged in front of the detectors 11 and 17.

In the configuration shown in FIG. 2, the tracking signal can be easily extracted even in a design in which the distance l between the images reproduced by the detectors 11 and 12 is extremely close (A'=1 m). Now, the astigmatic wavefront is converted into a hologram using an optical system as shown in FIG. As shown in figure a, a cylindrical lens 1o (cylindrical axis is in the x2 direction, x2 is X, which will be described later) is placed in the optical path in which the coherent parallel beam 13 of wavelength λ1 is focused by the condensing lens 60.

installed parallel to the ) to obtain linear focused beams 101 and 103 oriented in directions perpendicular to each other and a substantially circular beam 102 at an intermediate position thereof. It is also possible to correct the spherical aberration of the cylindrical lens 1o by inserting a slit 1o10 (slit widths ε and εa) into the beam 1o1. It is now assumed that the center of the beam 102 is at the origin on the front focal plane X1-Y1 coordinate plane of the Fourier transform lens 60.

This optical system converts an element similar to that conventionally used to generate astigmatism in an optical pickup optical system into a single hologram element, but the important thing here is that the first The circular beam 10 containing the astigmatism is Fourier-transformed to
By outputting the Fourier transform wavefront of 2 to the rear Fourier transform surface (ξ1-ma, expressed in coordinates) of the lens 6o and superimposing it with another plane wave that does not contain aberrations, a so-called lens Fourier transform type hologram element is created. It is in the configuration to create 12. The above reference wave is the Fourier transform lens 6
An aberration-free spherical wave that diverges from a predetermined position (point) 16 on the front focal plane of 0 is used. The reference wave can be easily obtained by converging the parallel beam 13 and the mutually coherent parallel beam 14 with the lens 16, but here, as a second component, A-A connecting the point 16 and the origin of xl-yl. 'The angle θ with the force direction Y1 axis is set to 460, so that the carrier wave frequency direction of the hologram is made to coincide with the 7-odd detector division direction, which will be described later.

Now, when the hologram element 12 recorded in this way is placed in an optical system as shown in FIG. On the rear focal plane (displayed by x2-Y2 coordinates), a reconstructed image 1021 of the wavefront including astigmatism and its conjugate image 1o22 are reproduced in a symmetrical positional relationship with respect to the X2-Y2 coordinate point, and each spot image Linear patterns 1011, 1031 and 1012° 1o32 in the horizontal (x2 axis direction) or vertical Y2 axis direction are obtained in the front and rear directions. Since they are conjugate wavefronts, one vertical linear image 1031 is located close to the lens 5, and the other horizontal horizontal image 1012 appears side by side.

As has been made clear from the above explanation, the reproduced wavefront from the hologram surface 121 in FIG. 2 forms a pair of astigmatism images on a pair of quadrant photodetectors 11.17 as shown in FIG. 3. FIG. 4 schematically illustrates the distribution of an astigmatism image obtained on one photodetector 11 when a multispectral light source as shown in FIG. 4A is used. The central spectrum λ corresponds to the focused point C and the defocused state d, respectively. A reconstructed image over a spectral width of ±Δλ is obtained almost symmetrically, and a focus control signal can be stably obtained. The focus control signal ε converts signal output components corresponding to each sector 111 to 114 of the quadrant photodetector into s1. s2. s3. As s4, control is executable.

The tracking signal ■ is calculated as ■=S2-84 with the tracking servo direction of the optical disk as the x'2 axis direction.
...It can be obtained from (2), but if the stability of the servo is given priority, as shown in FIG. It's okay.

FIG. 5 further describes the effects of the present invention obtained when the pair of photodetectors 11 and 17 are operated differentially in a second embodiment. However, for the sake of simplicity in figure a, the beam moves back and forth through the hologram element 12, but the principle of differentially detecting the control beam with a pair of photodetectors is exactly the same as in the case of figure 2. be.

Now, Figure 6 shows a pair of photodetectors 11°17
The principle of differential photoelectric conversion using a pair of conjugate images incident above is explained. Now, the wavelength components of the light source are λ before and after the center wavelength λ. Consider a distribution or fluctuation like ±Δλj. The figure shows the wavelength λ.

The wavelength component λ is placed on the photodetector 11°17 whose position is adjusted relative to the wavelength component λ. The image position of the astigmatic wavefront (at the time of focus) reproduced by +Δλj is explained. In the figure, the shift amount of the center position of each reproduced image from the intersection of the dividing lines of the four-part photodetector is equal to δ1, the distance between the hologram element 12 and the photoelectric conversion surface cloth is δ2, and the period corresponding to the carrier wave frequency of the hologram element 12 is p. Then, as a general example, d2=20. w, p=0.01m.

λ. =800nm=8X1 (>4n1.Δλ5
=20nm=2X10-"When ff, δ, ,=,,4
X10 ff 0 Also, the distance l between the centers of the photodetectors is 1=2d r=2d2SinT=1.6jf
fX2: 3.21nI Here, the wavefront (not shown in FIG. 6) diffracted from the hologram element 12 on the outward path can be determined by appropriately selecting the distance d1 between the collimating lens 9 and the condensing lens 5. , is suppressed to a virtually harmless amount of light.

In the configuration of the present invention, the setting of l may be approximate to the wavelength used due to the principle of differential signal detection.

The differential detection port path 180 in FIG.
The amount of light incident on 3 and 172, 114 and 171 is added in a relationship of 1=1 and combined and output to terminals 181 to 184.
If the output of 84 is again set as S1 to S4, Fig. 4 and (1)
Equivalent beam control is achieved when obtained using Eq.

Here, as shown in FIG. 3, the pair of conjugate images reproduced from the hologram element determine the area correspondence of the photodetector according to the principle that defocused images are reproduced with a positional relationship rotated by 900 degrees from each other.

In this way, light sources with arbitrary spectral distributions can be used, and in particular semiconductor lasers can always provide symmetrical and differential photoelectric conversion outputs even when there are spectral fluctuations due to temperature or current. This has great effects.

Another advantage of the present invention regarding mass production is that once recorded, the lattice pattern of the device can be transferred to a mold that can be easily reproduced, and furthermore, by making a replica using resin or glass material, a large number of uniform The device can be obtained at low cost and has great effects in designing and manufacturing optical information pickup devices. Since the pattern to be replicated is of the Fourier transform type, it is possible to blaze the mold by ion beam processing, etc., and the Fresnel zone plate blaze technology (Katai, Kubota, Nishida: 18th Micro-optics Research group lecture paper; vol 3 (1985) p, 3
Compared to 3), the diffraction efficiency of the replica element can be easily blazed. That is, the beam can be irradiated obliquely over the entire surface of the hologram consisting of a substantially parallel lattice pattern.

Although the embodiments of the present invention have been mainly described as beam control systems for optical disk optical systems, it can also be applied to optical systems with both recording and reproducing functions, and furthermore as optical information pickup devices in magneto-optical storage and reproducing systems. Of course. In addition, the recording density of the optical information storage medium is 6 to 1 in terms of pit size.
For optical cards, etc., which are good at a wavelength of about 0 μm or more, it is possible to use a quasi-monochromatic light source whose spectral width is relatively wider than that of conventional semiconductor lasers.

Effects of the Invention As is clear from the above explanation, the present invention uses a lens Fourier transform type hologram element in combination with other optical elements in the optical system of the OPU, and an astigmatic wavefront is recorded in the hologram. However, (1) the hologram itself does not generate aberration even if the wavefront is reproduced at a wavelength λ2 different from the hologram creation wavelength λ, and (2) even when the oscillation mode of the semiconductor light source is unstable and multispectral oscillation occurs. By setting the direction of the hologram carrier frequency in the direction of the dividing line of a pair of quadrant detectors, a correct control signal can be obtained, and differential detection is also possible if necessary. By integrating the front and back surfaces with the hologram surface, it is possible to effectively utilize the amount of light, suppress the return light to the light source, and enable stable signal detection, and furthermore, it is possible to make the optical system smaller and lighter.

(4) Similar to optical disks, hologram elements can be easily produced in large quantities as replicas through a transfer process from a mold that serves as a mask.

(5) Since a lens Fourier transform hologram is used that records a substantially parallel grating pattern, blaze processing using an ion beam or the like can be performed simultaneously on the entire surface in order to improve the diffraction efficiency of the element. A mask hologram mold can be manufactured.

(6) A characteristic of lens Fourier transform holograms is that the reconstructed image position remains unchanged with respect to translation of the hologram position, and only the rotational axis direction relative to the optical axis needs to be adjusted, and in the case of differential detection, the accuracy is also gradual. It is.

[Brief explanation of drawings]

1A and 1B are a schematic perspective view and a configuration diagram of an optical information pickup device showing one embodiment of the present invention, FIG. Figures 4a and 4d, which are principle diagrams of the present invention explaining examples of recording and reproducing an astigmatic wavefront, respectively, relate to an embodiment of the present invention for recording and reproducing an astigmatic wavefront. A conceptual diagram illustrating the focusing state of a beam to be reproduced; FIGS. 5a and 5b are a schematic perspective view of a pickup device using a hologram element of the present invention recording an astigmatic wavefront; and a conceptual diagram of a photoelectric conversion part; FIG. 6 is a schematic perspective view of a model optical system for explaining the present invention in detail, and FIGS. 7a and 7b are a conceptual diagram and a schematic perspective view of an optical pickup optical system using a conventional hologram element. 1...Semiconductor radar or equivalent coherent light source, 2...Collimating lens, 3...
...Polarizing beam splitter, 4... Quarter wavelength plate, 6... Condensing optical system, 6... Optical disk, 121, 12... Lens Fourier transform hologram (element), 8...Photodetector, 11.17...Photodetector, 10...
...Cylindrical lens, 18...Naifetsuji,
122...Polarized light selective reflection surface, 180...
・Differential detection circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure Jl Photo〒Itefta Figure 2 Figure 3 Figure 4 HΔλ−Imaichi Δλ← λρ Figure 5 Figure 6! Figure 7 (ku) (shi) Y

Claims (4)

[Claims] (1) A light source that emits a coherent beam in the infrared region or a wavelength range shorter than this region, a first optical system that converts the coherent beam into a substantially parallel beam, and selectively reflects a predetermined polarized wavefront of the coherent beam. a transparent substrate having a thin film medium on its first surface, a lens Fourier transform type hologram element with a predetermined wavefront disposed on the back side of the substrate, a quarter wavelength plate, and converging the coherent beam into a minute spot. a second optical system; a third optical system that Fourier-transforms a predetermined diffracted light from the hologram element in an optical path in which the coherent beam returns after being reflected by a predetermined optical storage medium via the second optical system; A first photoelectric conversion means disposed near the optical axis of the three-optical system, and a second photoelectric conversion means for detecting zero-order diffracted light transmitted through the hologram element in the far field or air field of the minute spot. An optical information pickup device characterized by: (2) The optical information pickup device according to claim 1, wherein the hologram element is a lens Fourier transform hologram with an astigmatic wavefront. (3) The optical information pickup device according to claim 1, wherein the hologram element is a lens Fourier transform hologram with a predetermined wavefront obtained by a knife-edge optical system. (4) The optical information pickup device according to claim 1, characterized in that a thin film medium that selectively reflects polarized light wavefronts and a lens Fourier transform hologram are integrated as front and back sides.